1. Field of the Invention
The present invention relates to sport fields and in particular, to artificial sports fields comprising artificial turf. The invention further relates to methods of construction for such sports fields and to water management and cooling thereof.
2. Description of the Related Art
Various artificial and semi-artificial sport field systems are known. Semi-artificial pitches usually involve integrating artificial grass blades into a soil base in which regular turf is allowed to grow. The soil base and its drainage arrangements may be otherwise similar to conventional natural grass pitches in order to ensure correct growth of the natural grass.
Fully artificial pitches have developed from first generation Astroturf™ to the present fourth generation systems, which attempt to combine all of the functions and characteristics of natural turf into a single product. In laying an artificial pitch, one fundamental requirement is an adequate base onto which the technical layers can be laid. A significant part of the overall cost of a new installation may lie in the preparation of the base. This should provide a guaranteed level of stability and drainage despite the fact that the underlying earth may vary considerably from one location or region to another.
Another characteristic of artificial sports fields is the difficulty of dealing with elevated temperatures e.g. in the presence of bright sunlight. The surface temperature of the field may rise significantly during the day, changing the characteristics of the technical layers and even giving off unpleasant odours. Play under these conditions may be compromised. Other sports fields may require irrigation before play can commence. In all cases, water management is a central issue that the designer must take into consideration when designing a sports field.
The term “water management” refers to four main areas, these being; drainage, irrigation, storage and attenuation.                Drainage may be defined as the removal of water from the pitch area to an exit point away from the field construction. Excessive water or surface flooding will adversely affect the play performance and can cause movement of infill materials.        Irrigation may be defined as the delivery of water onto the turf surface, by a system of pumps, pop-up sprinklers and water cannons. The purpose of adding hwater to the surface may be either as part of the playing characteristics of the surface, or to cool the turf surface. In hot, sunny condition, artificial turf systems may heat up, to the point that the surface becomes uncomfortable and energy depleting to the players.        Storage may be defined as providing a holding facility which allows water, drained from the pitch, to be used e.g. in the irrigation of the pitch. This may be in an underground reservoir or an above-ground lake. Storage is also linked to water attenuation.        Attenuation may be defined as the temporary storage of surface water in a suitable reservoir below ground level or in an above-ground lake. This reservoir or lake needs to be of sufficient size to accommodate the calculated run-off during peak periods of rainfall. The stored water may subsequently be gradually released in a controlled manner into a combined drainage system or watercourse, effectively reducing the risk of flooding.        
Drainage Systems
Traditional drainage methodology for artificial turf systems has been based around two main methods. The first been the vertical method and the other been the horizontal method.
Vertical Method: There are many variations and systems which come under the heading of vertical drainage, but the basic principal remains the same, that being a matrix or pattern of inter-connecting porous pipes, situated at the bottom of a porous rock base construction. The porous base construction is designed to remove water permeating down from the artificial turf system above, through the upper layers of the porous rock sub-base. These pipes lead the water off the playing area into ring main land drains or similar water drainage control systems.
These pitches may have a profile which assists water movement; this is known as slope or fall. There are various designs of slope and fall of the upper sport surface, such as; Crowned, Enveloped, and Tilted etc. the angles of fall tend to be between 0.4% and 1%. A key issue with porous stone bases is that they are required to have minimum depths of construction due to the limited amount of stone compaction which can be achieved. The target for any base construction is to achieve a certain tested value which ensures the structure has the correct structural integrity and load bearing capacities. As an example, one industry standard method of measuring these values is known as the Californian Bearing Ratio (CBR also defined according to BS 1377-2: 1990), which is expressed as a percentage value. For a sports field, the standard target CBR value after installation is a minimum of 30%. In order to achieve this value for a porous base construction a greater depth must be considered. This is because porous stone constructions contain void spaces in order to allow water to pass through the structure. As a result, the overall strength of the structure is not as high and the stone has more mobility, as compared to an equivalent non-porous base construction.
The presences of water within the porous stone construction also acts to destabilise the formation further and make the formation susceptible to frost heave. In order to address these issues, depending on local geo-logical and climate conditions, porous stone bases typically have a minimum depth of 300 mm.
Another issue is the requirement for expensive, specially graded rock types in order to create a porous base layer. This rock comes from virgin materials transported in by truck. For a standard porous pitch construction of 7500 m2 and a minimum stone depth of 300 mm it would require 2475 m3 of stone or 4200 tonnes. Given a standard truck can carry 20 tonnes this will require approx. 200 truckloads. Added to this is the number of trucks required to remove the existing sub-soil for re-location or landfill, being an additional 200 truckloads. This use of virgin materials and the requirement for 400 truck journeys, not only has a high cost but also a large environmental impact.
Horizontal Method: As explained above, the vertical drainage method relies on creating a porous base construction which allows water to percolate down through the various stone grades to the field drains below. The horizontal method uses a limited porosity or non-porous stone sub-base construction with no field drains below. The basic principal is that the water flows across the surface of the artificial turf, through the infill and in some cases, within a shock pad/substrate which allows water to flow horizontally, above the stone sub-base construction. In all such systems and methods, the pitches are designed with a slope or fall, (as explained above) in order to ensure adequate water flow. The water flowing off the sports field surface is then captured by drainage troughs or pipes installed around the perimeter of the turf installation. These pipes, themselves have a slope or fall in order to remove the water to main land drains or similar water drainage control systems.
The benefit of such systems over porous base constructions is the possible reduction of base construction depths, as these formations do not require void spaces and therefore can be compacted. The type of stone can have a wider particle size thereby reducing quarried stone costs. It is also possible to use recycled aggregates in such a base construction format, such as aggregates produced from recycled brick, concrete, asphalt etc. Because these surfaces are non-porous or have limited porosity, water has little effect on the structure of the base layer and therefore frost heave and destabilising effects are limited. In some cases it may be required to install an impermeable liner on top of the stone layer. In order to allow water to drain from the surface the pitch will be required to have a fall or slope. This slope can be constructed in a variety of ways, either in one direction or in multiple directions. The construction of such a slope requires skill from the pitch builder and influences ball behaviour of the finished turf surface. In addition a layer must be installed between the turf and the base construction in order for water to move freely down the slope to the perimeter drains. This layer can be provided by certain types of shock pads and/or geo-technical fabrics.
One horizontal drainage method for a sports pitch is suggested in WO2012138216 which uses a porous sub-grade course laid on an impermeable surface to transport water to the edge of the pitch, where it can be drained away by conventional drains. It is also proposed to use the sub-grade course as a containment e.g. for re-circulation to the playing surface as described below.
Irrigation
Standard irrigation systems rely on water from the mains supply which is pumped to water cannons or pop-up sprinklers located on or around the pitch. In some cases water captured from the pitch's drainage system is piped into storage facilities and recirculated back on to the pitch by means of water cannons or pop-up sprinklers. A filtration facility may also be required in this case. The use of the water is either as part of the turf system performance or for use as cooling during hot weather. For a full-sized hockey pitch from 12000 to 18000 liters may be required to wet the pitch prior to play.
All these methods rely on the same basic principal, whereby water is applied to the top of the turf system and the water gradually drains out of the turf structure. The water cannot be held in place for any medium or long-term time frame, therefore its influence on the turf system in aspects of performance or cooling has a limited short term effect. Because most artificial turf fibres and infill have little or no absorbent properties, water simply runs off through the turf system. The method of applying water by cannons or sprinklers is highly inefficient as much of the water applied is lost as mist before reaching the surface and the accuracy of water placement is difficult to control. The cost of such systems is also high, as well as the financial and environment impact of using water directly from mains supply.
Water Storage
If the requirement or legal obligation on a project is to reuse water captured from the pitch drainage system, or if this water cannot go directly into local drainage systems, then above ground or underground tanks or ponds may be constructed nearby the pitch. As stated above some designs will allow captured water to be irrigated back on to the pitch playing surface. The cost of installing tanks or ponds is high, and areas around the pitch must be set aside for such constructions. In many cases the solution is to dig a large hole and bury the tank. There is furthermore an issue of retained water becoming infected by microbes and algae's. The water can be treated to eliminate such infestations, however care must be taken to monitor and treat the retained water.
At the other extreme, in areas of low rainfall, sustainable water storage or rain water harvesting is a well know practice. These methods include ground and roof capture, sub-surface dykes and ground water recharge.
Attenuation
Sustainable urban drainage systems (SUDS) or low impact drainage systems (LID) are a developing concept that includes long term environmental and social factors in decisions about drainage. It takes account of the quantity and quality of runoff, as well as the amenity value of surface water in the urban environment. Many existing urban drainage systems can cause problems of flooding, pollution or damage to the environment and are not proving to be sustainable.
Built-up areas need to be drained to remove surface water. Traditionally this has been done using underground pipe systems designed for quantity i.e. preventing flooding locally by conveying the water away as quickly as possible. However, the alteration of natural flow patterns can lead to problems elsewhere in the catchment. Furthermore, amenity aspects such as water resources, community facilities, landscaping potential and provision of varied wildlife habitats have largely been ignored.
SUDS systems are designed to act as collection, storage and gradual release of rain water during and after large storm events. There are various types of urban systems in existence; from soak away pits, multiple sub-terrain chambers to open storage lakes etc. all of which are designed to ensure rainwater being collected from non-porous surfaces such as roads, roofs and car parks is directed, stored and then slowly released into the drainage system at a controlled, manageable flow rate.
The major drawback with these systems is the fact they need to be built in urban locations where space is at a premium. Systems which rely on soak away pits or areas of graded rock to hold water and then slowly disperse it by natural ground seepage are also prone to silting up due to sands, clays and fine dirt etc. slowly penetrating the formation.